The present invention relates to a control apparatus and method for operating a gas turbine, and, more particularly, to a control apparatus and a control method for a gas turbine of the type in which a control valve for controlling the combustion air flow rate is disposed in an air flow passage which supplies in from a compressor to the gas turbine, which control apparatus and method can control the air fuel ratio so that the emission of nitrogen oxides will be reduced.
It is important for a gas turbine combustor to reduce the emission of nitrogen oxides therefrom; therefore, it is important to control the air fuel ratio in the combustion section to of the gas turbine a suitable value. Many proposals concerning air fuel ratio control of gas turbine combustors have been provided and realized. An example of them is disclosed in JP A 4-186020, wherein a gas turbine combustor is provided with a changing mechanism for adjusting the opening area of an air intake port. The chemical emission spectrum of the flame in the main combustion zone is detected and the changing mechanism is controlled in closed loop on the basis of the detection result so that the air excess ratio (air fuel ratio) will be maintained at a value to be taken for a target value, in order to reduce the occurrence of nitrogen oxides and prevent the flames from being blown out.
Further, another method is disclosed in JP A 2-163423, in which the amount of air passing through a combustor is calculated from the compressor output pressure, the turbine inlet pressure, the turbine inlet temperature, the turbine outlet pressure, etc. and the air fuel ratio and fuel amount are controlled.
Further another method is disclosed in JP A 54-142410, in which each air flow rate in separated air flows into a high temperature section and a cold section of a step-up apparatus in a turbo fun engine is calculated by using measured engine parameters and known parameters, and a schedule control of a high temperature section fuel air ratio and a cold section fuel air ratio is effected, based on the calculated values.
In a conventional air fuel ratio control, calculation of an air amount is carried out, based on the following technical knowledge:
1) The air amount is proportional to the rotating speed of the turbine; PA1 2) The air fuel ratio is a function of air pressure and the electric generator output; PA1 3) The air flow amount is represented by a difference between a compressor intake air amount and an extraction air amount; PA1 4) The air amount is the function of the compressor output pressure, the turbine inlet pressure and the temperature. PA1 5) The turbine inlet temperature is a function of the air fuel ratio and the turbine rotating speed; PA1 6) The turbine inlet temperature is a function of the air amount and the exhaust gas temperature; and PA1 7) The turbine inlet temperature is the function of a stationary blade temperature and the change ratio of the temperature.
Further, calculation of a turbine inlet temperature, which is one of the main parameters of the combustion conditions is carried out on the basis of the following premises:
In general, in closed loop control of the air fuel ratio, the response is slow when the turbine load is changed, since a change in the air amount will not follow a change in an fuel amount, and as a result, the air fuel ratio changes, so that emission of nitrogen oxides increases, or misfire and after burn take place.
Further, if a calculation based only on the above items 1) to 4) is employed, an air amount to be supplied to the combustion zone of the combustor can not be correctly estimated. Namely, these value are not measured values of an air amount to be supplied actually to the combustion section, but are values calculated using an air amount, pressure and temperature at the compressor section or turbine section as parameters. In the actual combustion, there exists an amount of bypass air bypassing the combustion zone without being subjected to combustion and an amount of cooling air, and a change in these air amounts changes the amount of combustion air supplied to the combustion zone. Therefore, it is difficult to estimate correctly the amount of air to be supplied to the combustion zone by the above-mentioned calculation. Accordingly, usually, the air fuel ratio is set to a little smaller value in view of the calculation error in the air amount, resulting in insufficient reduction in the emission of nitrogen oxides.
Further, if only the above calculation of an air amount is employed it is difficult to obtain an air amount for each combustor in a gas turbine provided with a plurality of combustors and to obtain air amounts in local places of each combustor. Therefore, it is difficult to solve an imbalance in air fuel ratio due to a difference in air amount between the combustors and a difference in air amounts in the local places of each combustor. In this case also, the air fuel ratio is set to a smaller value in view of the above-mentioned differences in fuel amount, so that the reduction in the emission of nitrogen oxides is insufficient.
The reduction in the emission of nitrogen oxides is more remarkable in the case in which a calculation result of turbine inlet temperature as shown in the above items 5) to 7) is used for calculation of an air amount.
As disclosed in JP A 54-142410, in the method in which each of the flow rates in the divided flows into the high temperature section and the cold section of the step-up apparatus is obtained by calculation, and a scheduled control of the high temperature section fuel air ratio and the cold section fuel air ratio is effected, based on the calculated flow rates, the desired air amounts in local places of the combustor can be obtained and each air amount can be controlled locally. Therefore, the method may suggest a method of eliminating any imbalance in the air fuel ratios among local places of the combustor. In this case also, however, an air amount is calculated by using measured engine parameters and known parameters, so that the disadvantage that the obtained air amount is not necessarily coincident with an air amount really supplied can not be avoided.
Further, in the case where an air amount is calculated the air fuel ratio is controlled on the basis of the calculation result, in order to effect a correct control without any delay, it is necessary to establish a system environment which is easy to model. It may be relatively easy for a turbo fan engine of the type which is disclosed in JP A 54-142410. However, as disclosed in JP A 4-186020 or in JP A 2-33419, for example, in a gas turbine of the type in which an air flow control valve for controlling a flow rate of combustion air is disposed in an air flow passage which supplies from a compressor to a gas turbine, air distribution patterns may occur which are not easy to anticipate or are difficult to model, so that precise air amount calculation is impossible and the construction of a satisfactory control system is very difficult.